Yes that probably is Messor capitatus.
The two common species of Messor that live in south europa are really hard to tell apart by the queens.
However when the first workers appear it is easy to tell since Messor capitatus has black workers and Messor wasmanni, the other Messor common in south europe, has a red thorax.
In rare occasions I saw Messor wasmanni workers that were almost completley black but you still could tell them apart.
The queen of Messor capitatus might not be that intimidatingly big but the grown majors are.
The reason why they are called Messor capitatus (I think it comes from caput which means head) is because of the ridiculous size of their head in relation to their body. This picture was taken by a friend of mine during our Croatia expedition in late 2010.

When you measure the ant using the structure of the box that you can see on the side you can compare it to this Camponotus aethiops Major;
Hyperlinks sind nur für registrierte Nutzer sichtbarIt is nearly same size so it should be the biggest Messor in europe, however it looks the biggest because of it´s gigantic head. Also some of the Majors aren´t able to walk because of the head, they stay inside and just crack open big seeds or rarley come out to cut grass that grows on the streets of the ants. By the way cutting away gras is one of the main reasons why the majors of those species get this big, there aren´t very many hard seeds to crack and they aren´t fit for fighting anymore.